Everyday tasks, such as handling and manipulating objects, may pose difficult challenges for survivors of stroke. The way in which our brain controls arm movements is likely to be shaped by our experience of interactions with objects and their mechanical and dynamic properties. Objects that we handle can be divided into two broad classes: rigid objects, such as a hammer, and objects with internal dynamics, such as a cup of water. Free reaching and interactions with rigid objects have been studied extensively, while interactions with objects with internal dynamics have not been adequately explored. Experiments presented in this proposal will focus on understanding how movements of the arm are controlled and coordinated when manipulating objects with internal dynamics. Aspects from modern control theory will be used to design and classify dynamic systems with which subjects interact. The experiments presented are centered on the hypothesis that healthy subjects learn to control objects with internal dynamics by developing an internal representation of their dynamics. Further, it is hypothesized that the ability to form an internal representation of these systems is contingent upon intrinsic object properties such as its controllability and observability.